1. Field of the Invention
The invention relates to an electric power steering system.
2. Discussion of Background
Generally, electric power steering systems (EPSs) include a motor as a drive source, a torque sensor, and a control device. The torque sensor generates a sensor signal on the basis of twisting of a torsion bar provided in the intermediate portion of a steering shaft. The control device detects a steering torque on the basis of the sensor signal and controls the torque of the motor so that a suitable assist force is applied to a steering system according to the steering torque. Therefore, in the EPS, in order to obtain the suitable assist force, it is necessary to stably detect the steering torque with high accuracy.
For example, an EPS including a torque sensor that outputs two sensor signals is disclosed in Japanese Patent Application Publication No. 2006-248354 (JP 2006-248354 A). In the EPS, when abnormality has occurred in one sensor signal out of the two sensor signals, the assist force can continue to be applied to the steering system by using the remaining normal sensor signal.
In the EPS in JP 2006-248354 A, in a case where there is a possibility that abnormality has occurred also in the remaining normal sensor signal, if a state where the remaining sensor signal is suspected to be abnormal continues for a predetermined period of time, it is determined that the remaining sensor signal is abnormal to stop the application of the assist force to the steering system. However, since the EPS in JP 2006-248354 A detects the abnormality of the torque sensor on the basis of the amount of fluctuation of the sensor signal, the abnormality of the torque sensor caused by other factors may not be detected.
For example, when the abnormality of the remaining sensor signal is a reverse gain failure in which the plus or minus of the detected value of the steering torque becomes reverse to the original sign, the abnormality cannot be detected and an assist force in a direction opposite to a steering direction is generated in the EPS in JP 2006-248354 A. In the EPS in JP 2006-248354 A, self-excited vibration may occur due to the so-called reverse assist. That is, when steering to the right is performed, assist in a direction toward the left side opposite to the original assist direction is provided. If the steering is turned to the left by being pulled with the assist, assist toward the right side is provided. Thus, vibration may be caused in the steering by the assist in the original assist direction and the assist in the direction opposite to the original assist direction being repeatedly provided due to the driver's steering operation. This may occur even in an EPS including a torque sensor that outputs one sensor signal.